1. Field of the Invention
This invention relates to a new photoconducting material. More particularly, it relates to a photoconducting sulfur- and hydrogen-doped amorphous carbon.
2. Description of the Prior Art
Amorphous semiconductors have long been employed as the photoconducting element in the xerographic reproduction process, with amorphous selenium being the classic photoconducting material. For xerographic applications, the photoconducting material must have a very low dark conductivity (less than about 10.sup.-13 ohm.sup.-1 cm.sup.-1) to prevent self-discharge of the unilluminated regions of the material, a large photoconductivity to rapidly discharge illuminated regions of the material and a high dielectric strength to minimize the thickness of the material which is required. A very hard material is also desirable, as the material is abraded by the paper and toner that it comes in contact with during the reproduction process.
Amorphous silicon which incorporates up to about 30 atom percent hydrogen, often referred to as hydrogenated amorphous silicon, is currently being developed as an alternative to amorphous selenium for xerographic applications because it is far harder and should have a service life which is about ten times longer (about 50,000 copies for an amorphous selenium coating versus about 500,000 to 1,000,000 copies for a coating of hydrogenated amorphous silicon). Unfortunately, the dark conductivity of hydrogenated amorphous silicon is too high for it to be used alone and, as a result, complicated multilayer films incorporating blocking contacts must be employed to prevent self-discharge in the dark.
Carbon films have been produced by a variety of vacuum deposition techniques which include electron beam vacuum evaporation, radio frequency sputtering, radio-frequency plasma decomposition of hydrocarbon gases, direct current glow discharge in hydrocarbon gases, coaxialpulsed plasma acceleration using methane gas, vacuum arc deposition using a graphite cathode, ion beam deposition with argon and hydrocarbon scission fragment ions, and deposition using pure carbon ion beams. Typically, however, sputtering, electron beam evaporation and plasma deposition are the most convenient techniques for the preparation of these films. When produced by decomposition of a hydrogen-containing starting material, such as a hydrocarbon, the carbon films typically contain significant amounts of hydrogen.
The above-described carbon films are very hard and, typically, have a Mohs hardness of about 6, a low dark conductivity, and a high dielectric strength. In addition, the optical bandgap of such films can be varied from less than 1 eV to greater than 2.5 eV by varying the preparative conditions. The carbon in these films is unlike graphite and has been described in the scientific literature as diamond-like or amorphous. For the purposes of this application, all substantially nongraphitic carbon which is produced by vacuum deposition techniques is hereinafter referred to as amorphous carbon.
Various studies which have utilized techniques such as X-ray diffraction, electron microscopy and electron diffraction have demonstrated that the carbon which is produced by vacuum deposition techniques is essentially amorphous in character. Unlike graphitic carbon, which is an excellent conductor of electricity, amorphous carbon is a semiconductor with a relatively high resistivity which decreases with increasing temperature. Finally, amorphous carbon is essentially transparent to red and infrared light whereas graphitic carbon is not. These properties suggest that a significant fraction of the carbon atoms in amorphous carbon are four-coordinate as in diamond rather than three-coordinate as in graphite.
The glow discharge initiated polymerization of various sulfur-containing organic monomers has been described by A. Bradley and P. Hammes, J. Eletrochem. Soc., Vol. 110, pp. 15-22 and 543-548 (1963). The monomers described include thiourea, thianthrene, thioacetamide and thiophene, and the polymerization was carried out using an alternating voltage in the ultra-audio frequency range (10 to 50 kilohertz) and at a relatively high pressure (about 1 torr). The films resulting from polymerization of thioacetamide and thianthrene were reported to possess a small amount of photoconductivity [J. Electrochem. Soc., Vol. 110, 543 (1963)]. More specifically, the ratio of the photoconductivity (corrected to an incident light intensity of 90 milliwatts/cm.sup.2 using the intensity dependence quoted by the authors) to the dark conductivity for the polymers derived from thianthrene and thioacetamide was only 138 and 250, respectively.
The glow discharge initiated polymerization of carbon disulfide (CS.sub.2) has also been described by Y. Asano, Jap. J. Appl. Phys., Vol. 22, 1618-1622 (1983). The polymerization described by Asano was carried out at a pressure in the range from about 2.times.10.sup.-2 to about 6.times.10.sup.-2 torr in a glow discharge which was sustained by an Rf generator operating at 13.56 megahertz. The sulfur to carbon ratio (S/C) in the resulting polymer was a function of Rf power and substrate temperature, and ranged from 0.16 to 14. In addition, the films were found to be photoconducting. More specifically, the ratio of the photoconductivity (corrected to an incident light intensity of 90 milliwatts/cm.sup.2 using the intensity dependence quoted by the author) to the dark conductivity for such a polymer having an S/C of 4.0 was 1429. At reduced values of S/C, this ratio of photoconductivity to dark conductivity was reported to decrease and reached a value of 237 at an S/C of 1.8.